The present invention relates generally to a paper cassette unit, and more particularly to a paper cassette unit which is detachably mounted in an image forming system and holds sheets of paper on which an image formed by the image forming system is recorded. This paper cassette unit is applicable to copiers, printers, facsimile machines, or the like.
Conventionally, an image forming system uses sheets of paper on which an image, for example, one reproduced from an original document, is recorded. A paper cassette unit includes a paper cassette in which the sheets of paper are accommodated, and is detachably mounted in the image forming system. When replenishment or exchange of sheets of paper is necessary an operator demounts the paper cassette unit from the image forming system, an replenishes the paper cassette with new sheets of paper, or replaces the paper cassette unit with another such unit containing sheets of paper of a different size.
FIG. 1 shows a conventional paper cassette unit detachably mounted in an image forming system. This paper cassette unit 1 shown in FIG. 1 includes a cassette cover 2 for covering an opening portion 1a of a paper cassette so as to keep the sheets of paper in the paper cassette free from external moisture or water. When the paper cassette unit is replenished with new sheets of paper or replaced with another paper cassette unit, the paper cassette unit 1 with the attached cassette cover 1a is demounted from the image forming system. The cassette cover 2 is detached from the paper cassette unit 1, the opening 1a of the paper cassette being open to the exterior. Once the paper replenishment or exchange is finished, the cassette cover 2 has to be attached to the paper cassette unit 1, in order to cover the opening 1a, and the paper cassette unit 1 with the attached cassette cover 2 has to be mounted again in the image forming system.
A paper cassette unit capable of holding plural sizes of sheets of paper requires a size detection member for giving information on paper size change to the image forming system when the previous sheets of paper in the paper cassette are replaced by new sheets of paper of a different size. The paper cassette unit of this type includes a paper cassette provided with a paper-size detection actuator and a paper size indicator, both separately attached to the paper cassette. When a paper size is changed, the paper-size detection actuator and the paper-size indicator in the paper cassette unit have to be also replaced by different ones. The paper-size detection actuator uses a width guide member formed in the paper cassette for detecting paper size, and this width guide member slides freely in the paper cassette in a direction of the width of inserted paper. Thus, the paper-size detection actuator can detect the size of the new paper in the paper cassette by means of the position at which the width guide member is set.
In a case in which the above conventional paper cassette unit is mounted in the image forming system, an operator may forget to attach the cassette cover 2 to the paper cassette when the replenishment or exchange of sheets of paper is carried out. When the paper cassette unit 1 is inadvertently mounted in the image forming system, with no cassette cover 2, the sheets of paper in the paper cassette are subjected to external moisture. When the moist paper dries, it will be in a distorted or wavy condition, If such paper is used, in the image forming system, there is a problem in that two or more sheets of paper are erroneously fed by a paper feeding part of the image forming system, poor image transfer takes place at an image transfer part of the system, or the paper becomes corrugated due to heat generated by a fixing part of the system. Thus, the use of defective paper will cause reduce the quality of the performance of the image forming system. Also, with the conventional paper cassette unit, an operator is apt to perform unintentional operation when the paper-size detection actuator and the paper-size indicator are exchanged for changing paper size. Moreover, if the operator exchanged only one of the actuator and the indicator and did not exchange the other, or forgot to do so, a malfunction of the image forming system would occur due to the disparity, between the actuator and the indicator used therein, with respect to indicated paper size.
There is also a problem in that the above described paper cassette unit having the paper-size detection actuator and using the width guide member for paper size detection is of relatively large size and the cost of manufacture is high.